This invention relates to a method and an apparatus for automatic train control in a digitally controlled model railroad system. In contrast to conventionally controlled or analog operated model railroads, in digitally controlled systems each locomotive or motor vehicle has its own individual address. In digitally controlled systems, too, it is possible for a locomotive to stop automatically in front of a railway signal that is showing “Stop”. For this purpose, the operating voltage is turned off in a stop section that is galvanically isolated from the rest of the track. However, the locomotive can then no longer be controlled by the digital control system, because it can no longer receive its control information.
To solve this problem, special strings of digits were inserted into the digital signal for such a stop section, for example, strings of digits that can be detected and analyzed by each locomotive that is equipped with digital receivers. However, necessary provisions on the track site to enable insertion of such particular digits ahead of each signal are quite extensive and therefore result in high costs.
Another and significantly simpler method is to make the digital signal asymmetric for such a stop section and to evaluate this asymmetry information in the locomotive decoder. Normally, the digital signal in almost all digital systems consists of an AC voltage having negative and positive components of equal amplitude, i.e. one that is symmetric. The advantage of an apparatus that utilizes this method of asymmetry in the digital information consists of its simplicity. On the track site, all that is needed are a few rectifier diodes, and on the decoder site trivial comparator circuits.
Whereas the asymmetric system described above has a very simple construction, it exhibits the following disadvantages. The large-scale industrial trains recognize, in addition to a stop at a signal, two additional conditions that are not implemented in the conventional asymmetric system described above. The first of these conditions is that the stop signal does not apply to a train that is approaching the railway signal from its back side. The second is that in addition to the “stop” information, there is also a “restricted speed” information. Nor can this method be used if the digital system has the capability of simultaneously operating a conventional (analog) locomotive. That is because the track voltage is generally transmitted to the comparator of a locomotive detector via an RC circuit. Because of the different pulse lengths that are inherent to conventional or analog operation (see DE 30 25 035), the locomotive decoder would already detect an asymmetry for this reason.